World number three Serena Williams and fourth-ranked Ana Ivanovic withdrew from the WTA Championships on Friday.
Williams, who also pulled out last year after playing just one set, blamed a sore stomach muscle.
Ivanovic, who is suffering from a virus and after losing her first two matches could not qualify for the semi-finals, defaulted her last match against Russian Svetlana Kuznetsova.
Although Serena won just one game in the last two sets of her defeat by sister Venus on Thursday, the American said she only felt the problem develop back at her hotel.
"I felt I didn't serve well, I had a lot of double faults," Serena told reporters. "Looking back I think maybe that's why I was doing it because I didn't feel it (the injury) at the time.
"I think I might have been over-compensating but the moment I got home I did feel pain. When I woke up around 2pm I was still in pain just getting out of bed."
After beating Dinara Safina and losing to Venus, Serena needed to defeat Olympic champion Elena Dementieva on Friday to qualify for the last four.
Now it will be Dementieva who joins Venus as a qualifier from the maroon group while Jelena Jankovic and Vera Zvonareva progress from the white group.
Zvonareva remained undefeated on Friday, adding a 2-6, 6-3, 6-4 victory over Jankovic to two previous wins.
In Saturday's semis, Zvonareva meets Dementieva and Jankovic faces Venus.
Ivanovic received medical attention for dizziness in both her matches this week and said her virus had become worse.
"The other night was a very intensive and long match and I started to feel worse," said the Serb. "Yesterday I didn't feel good and today even worse.
"I tried to hit yesterday. I was on the court for about 10 minutes and as long as I didn't move it was okay. But if I tried to hit a little bit harder or to move to the side it was impossible."
